1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a method for controlling luminance of an edge type backlight unit having a plurality of lamps.
2. Discussion of the Related Art
Cathode ray tubes (CRT) have been widely used for television monitors, measuring machines, and information terminals. However, CRT technology cannot satisfy the increasing demand for miniaturized, light weight displays due to the size and weight of the CRT itself. As a result, alternative technologies such as liquid crystal display (LCD) devices that use an electric field optical effect, plasma display panels (PDP) that use a gas discharge, and electroluminescence display (ELD) devices that use an electric field luminous effect to display images have been developed to replace the CRT.
Due to its advantageous characteristics such as compact size, light weight and low power consumption, the LCD device has been actively studied to make it suitable for ultra-thin flat display devices, for example, monitors for spacecrafts, aircrafts, notebook computers, laptop computers, desktop computers and large-sized display devices. As a result, the demand for LCD devices continuous to increase.
Most LCD devices display images by controlling the transmittance of light from internal, external, or a combination of internal and external light sources. In general, backlight units used as internal light sources for LCD devices are classified into two types, the direct type and the edge type, according to the arrangement of the fluorescent lamps within the backlight unit.
In edge type backlight units, a lamp unit is provided at an edge or side of a light-guiding plate. The lamp unit includes a lamp that emits light, a lamp holder inserted onto both ends of the lamp to protect the lamp, and a reflective sheet attached to the light-guiding plate and surrounding the circumference of the lamp, to reflect the light emitted from the lamp towards the light-guiding plate. The edge type backlight unit is generally used in smaller sized LCD devices, for example, laptop and desktop computer monitors because of it has great uniformity of light, long life span, and a thin profile.
In the direct type backlight unit, a plurality of lamps are formed in a line on a lower surface of a light-diffusion sheet, whereby an entire surface of the LCD panel is directly illuminated with the light. Because the direct type backlight unit has greater light efficiency compared to the edge type backlight unit, it is generally used for the larger-sized LCD devices, for example 20 or more inches, requiring high luminance.
In the direct type LCD device, a plurality of lamps are provided underneath a screen of the LCD panel. Accordingly, if one of the lamps is turned off due to trouble with the lamp or the end of the lamp's life, the portion of the screen corresponding to the turned-off lamp becomes darker than the surrounding portions of the screen. In contrast, in the edge type backlight unit, when lamp is turned off, the over all luminance of the backlight is decreased slightly as opposed to a stark decrease in only a portion of the screen. In this respect, it is necessary for the direct type LCD device to have a simple structure suitable for disassemble and assemble of the lamp unit.
FIG. 1 illustrates an edge type backlight unit according to the related art. As illustrated in FIG. 1, the edge type backlight unit includes a lamp 10 positioned at opposite sides of a light-guiding plate 11, a light-diffusion sheet 12, a prism sheet 13, a housing structure 15, and a lower reflective sheet 16. An LCD panel 14 displays images thereon by controlling the transmittance of light emitted from the lamps 10. The light-guiding plate 11 guides the incident light, emitted from the lamps 10, towards the LCD panel 14, through the diffusion sheet 12 and the prism sheet 13. The light-diffusion sheet 12 diffuses the light guided by the light-guiding plate 11 to a predetermined angle, and the prism sheet 13 concentrates the diffused light, and sends the concentrated light to the rear of the LCD panel 14. Also, the housing structure 15 is positioned below the light-guiding plate 11. To minimize the loss of light, the lower reflective sheet 16 reflects the light progressing toward the housing structure 15 to the LCD panel 14.
The edge type backlight unit according to the related art further includes lamp reflective sheets 18 and lamp holders 17. Each of the lamp reflective sheets 18 covers the lamp 10 except for the surface incident to the light-guiding plate 11, thereby decreasing the loss of light incident on the light-incidence surface of the light-guiding plate 11 from the lamp 10. In addition, the lamp holders 17 are positioned at each end of the lamp 10 to position the lamp 10 and to maintain a predetermined interval between the light-incidence surface of the light-guiding plate 11 and the lamp 10.
In the aforementioned edge type backlight unit, the plurality of light-diffusion sheets 12 and prism sheets 13 may be deposited as need.
When the edge type backlight unit according to the related art is used in a monitor, a lamp 10 is positioned at opposite sides of the light-guiding plate 11. However, if high luminance is required, a plurality of lamps 10 may be positioned at opposite sides of the light-guiding plate 11 depending to the size of the LCD panel 14. In either case, all the lamps 10 are simultaneously turned on or off according to the related art.
In the related art edge type backlight unit, the plurality of lamps 10, arranged at opposite sides of the light-guiding plate 11 are operated simultaneously, thereby controlling the entire luminance of the LCD panel 14. As a result, the related art cannot partially control the luminance of the LCD panel according to the displayed images.